a.rb

class Hash
  def stringify_keys!
    keys.each do |key|
      self[key.to_s] = delete(key)
    end
    self
  end

  def jsonify_necessary_values!
    values.each do |key, value|
      self[key] = value.to_json if %(key startkey endkey).include?(key)
    end
    self
  end
end

module Addressable
  class URI
    alias :orig_query_values= :query_values=

    def query_values=(params)
      params.stringify_keys!
      params.jsonify_necessary_values!
      self.orig_query_values = params
    end
  end
end

class String
  def to_uri(params={})
    Addressable::URI.parse(self).tap do |uri|
      uri.query_values = params
    end
  end
end

core_ext.rb

require 'addressable/uri'

class Object
  def tap
    yield(self)
    self
  end
end

class Hash
  def stringify_keys_and_jsonify_values!(values=%w(key startkey endkey))
    keys.each do |key|
      value = self.delete(key)
      value = value.to_json if values.include?(key.to_s)
      self[key.to_s] = value
    end
    self
  end
end

module Addressable
  class URI
    alias :orig_query_values= :query_values=

    def query_values=(params)
      params.stringify_keys_and_jsonify_values!
      self.orig_query_values = params
    end
  end
end

class String
  def to_uri(params={})
    Addressable::URI.parse(self).tap { |u| u.query_values = params }
  end
end

z.rb

module Addressable
  class URI
    alias :orig_query_values= :query_values=

    def query_values=(params)
      keys.each do |key|
        self[key.to_s] = delete(key).to_json if %(key startkey endkey).include?(key.to_s)
      end
      self.orig_query_values = params
    end
  end
end

class String
  def to_uri(params={})
    Addressable::URI.parse(self).tap do |uri|
      uri.query_values = params
    end
  end
end